Increased insulin secretory capacity but decreased insulin sensitivity after correction of iron overload by phlebotomy in hereditary haemochromatosis.
AIMS/HYPOTHESIS: We recently demonstrated that humans with hereditary haemochromatosis have decreased insulin secretory capacity with a compensatory increase in insulin sensitivity. We therefore determined how these measures change after correction of tissue iron overload. SUBJECTS AND METHODS: Five non-diabetic subjects who had been studied previously at the time of initial diagnosis by means of the OGTT and frequently sampled intravenous glucose tolerance tests (FSIVGTT) underwent phlebotomy to normalise their serum ferritin. After normalisation of ferritin they were studied again (33+/-4 months after the initial studies) by OGTT and FSIVGTT. RESULTS: Normalisation of tissue iron stores resulted in an average 1.8-fold increase in the integrated area under the insulin curve during OGTT (p<0.0001), but no significant change in the area under the glucose curve (10% decrease, p=0.32). After phlebotomy, there was a 2.2-fold increase in insulin secretory capacity as determined by FSIVGTT (acute insulin response to glucose [AIRg], p<0.02) but a concomitant 70% fall in insulin sensitivity (Si, p<0.05). The disposition index (AIRgxSi) was unchanged (5% increase, p=0.90). BMI and fasting glucose were unchanged. At the time of diagnosis of haemochromatosis, four of the subjects had IGT. After normalisation of ferritin, two achieved NGT and two remained with IGT, despite 2.5- and 3.7-fold increases in insulin secretory capacity. CONCLUSIONS/INTERPRETATION: Insulin secretory capacity improves after normalisation of iron stores in subjects with hereditary haemochromatosis. Glucose tolerance status improves incompletely because of decreased insulin sensitivity after phlebotomy. We conclude that tissue iron levels are an important determinant of insulin secretion and insulin action.